Mass spectrometers are known instruments used to analyze the mass spectrum of a beam of ions generated from a specimen of material to be analyzed which is ionised in for example a plasma. The basic components of a standard mass spectrometer are an ion source, a mass analyzer such as a quadrapole, an interface for directing a beam of ions from the source to an inlet aperture of the analyzer, and a detector to detect the ions that pass through the analyzer. The interface includes a sampling cone through an aperture in which a beam of ions leaves the ion source.
In some mass spectrometers the cone aperture and the inlet aperture to the mass analyzer are in line on the axis of the analyzer, the analyze ions being focused using standard optics into a beam which travels along a straight path from the cone to the analyzer. Unwanted particles, such as neutral particles and photons, are removed from the analyze beam, before it enters the analyzer, using filters such as a Bessel Box. Such a system essentially comprises stops on the axis of the system and utilises standard optics to deflect the wanted ions around the stops before they are focused along the axis of the spectrometer. Such systems reduce, but do not eliminate, the number of unwanted particles that pass through the analyzer to be registered by the detector as "noise". It is also-known to position the detector off the axis of the analyzer and use standard optics to deflect ions exiting the analyzer towards the detector. Unwanted particles continue along the axis of the spectrometer.
Mass spectrometers of this general type have the disadvantage that they are often found to exhibit mass bias effects. The general form of this bias is a severe loss of transmission through the spectrometer of light elements. This is obviously an unsatisfactory situation in which to can carry out analytical measurements. These bias effects are essentially electrostatic in nature and arise from two distinct sources. Firstly, the optics of the Bessel Box, which bend wanted ions around stops, deflect lower energy particles to a greater degree than higher energy particles so that only a narrow energy band of particles which are of interest will pass through to the analyzer. As all particles enter the system from the source with similar velocities their energy will be dependent upon their mass. Hence only one mass of particles from the analyze ion beam will push through to the analyzer. The second source of mass bias arises from so called "space charge" effects. Positively charged ions in the analyze beam will repel each other with a normal Coulomb force. The effect of this is that some positive charged particles will be lost from the beam. Again it is the low mass low energy particles that are more likely to be lost.
To combat this problem, mass spectrometers have been built incorporating an accelerating electrode. This is typically in the form of an apertured cone situated behind standard sampling systems in the interface between the ion source and the mass analyzer. The ion beam from the source passes through the aperture in the cone which provides a strongly accelerated and convergent electrostatic field which acts upon the positively charged ions. The effect of this field is to squeeze incoming ions into a tight beam and rapidly transfer them from the sampling inlet to the analyzer inlet aperture, thus reducing any loss due to space charge effects. Such systems generally further comprise simple x, y deflection systems situated behind the accelerator cone to deflect the ion beam into the inlet aperture of a quadrapole mass analyzer which is situated off the axis of the ion beam as it emerges from the source. This dispenses with the need to employ for example a Bessel Box, as neutrals will not be bent by the x, y deflector and therefore will continue along the axis of the system and not enter the off axis mass analyzer. Systems of this sort have been shown to be effective in reducing noise levels and avoiding mass bias effects.
Most of the noise still picked up by detectors in such systems is generally attributed to stray photons. However, tests have shown that many unwanted counts result from neutral particles entering the mass analyzer. The source of these neutral particles may be positive ions in the analyze beam which collide with residual gas particles in the system and thus undergo a process of charge exchange resulting in the formation of neutral particles.
Systems such as the one described above which employ a particle accelerating electrode are particularly susceptible to this problem This is because the ions are decelerated in the vicinity of the inlet aperture to the mass analyzer, where there exists a relatively high pressure region in which the likelihood of collisions between analyze ions and residual gas particles is increased. Because this takes place close to the inlet aperture to the analyzer almost all the neutral particles resulting from charge exchange in this region will enter the analyzer. It is an object of the present invention to obviate or mitigate these disadvantages.